1. Field of the Invention:
The present invention relates to a luminance modulation system for a DC type image display device wherein radiating elements or light quantity control elements are matrix- arranged at each picture element for composing an image.
2. Description of the Prior Art:
An image display device of such matrix display type has various problems in performance, and one of the problems is an insufficient contrast of the image to be displayed. It is cuased by a deficiency of the highest luminance which can be displayed by such a device, but according to the experiment carried by the present inventor, it has been found that this insufficient contrast is particularly remarkable in an image display device using a gas discharge cell as a radiating element, because, in the gas discharge cell, the lowest luminance is determined by the minimum discharge current necessary for maintaining the discharge in a conventional driving system. Moreover, in the gas discharge cell, this minimum discharge current becomes fairly large and the luminance of the image displayed by this mininum discharge current becomes considerably high, so that the contrast ratio of the lowest luminance to the highest luminance cannot make a sufficiently large value. In order to increase this contrast ratio, various methods have been considered and one of them is a method of lowering the lowest luminance to be displayed by controlling the operating time of the radiating element or the light quantity control element for composing a picture element. If this method is applied to the image display device of the gas discharge type merely as it is, the instant of starting the discharge in the discharge cell is delayed and in case of displaying the image by the gas discharge, at least a time for maintaining the discharge which is larger than the delay time is required, so that the ratio of the minimum discharge time to the maximum discharge time, i.e., the contrast ratio, connot sufficiently be made large.
As to the image display device of the gas discharge type, the present inventor has previously proposed a method of increasing a contrast ratio of the image to be displayed by duty time-modulating a discharge current of the gas discharge cell in a low luninance region and by amplitude-modulating, i.e., level- modulating it in a high luninance region, but in this method, it is necessary to provide two systems of a duty time-modulation circuit and an amplitude modulation circuit as a circuit for driving the discharge cell, either one of both modulation circuits being operated in accordance with the level of an image signal. Besides, it is required that the highest level of the output of the duty time-modulation circuit operated for the low luminance region coincides precisely with the lowest level of the output of the amplitude modulation circuit operated for the highest luminance region. If not so, the luminance characteristics of the image displayed by the gas discharge cell become discontinuous before and after switching these two modulation circuits with each other, and when the image signal is varied from the low level to the high level, the luminance of the discharge cell is sometimes varied in the darker direction. In this method as described above, the modulation circuits of two systems are required, and a switching circuit is also required for automatically selecting either one of these two systems in accordance with the level of the image signal, so that the construction of the whole circuit for driving the discharge cell becomes very complicated. Consequently unevenness of the characteristics at every driving channel is increased and the picture quality of the displayed image is lowered by shading of the radiation.